In recent years, different types of communication networks have been developed to carry various types of information. Communication networks typically make use of one of two well-established transmission mechanisms, namely the circuit switched transfer and the packet-switched transfer. The older networks like telephone networks for voice communication are primarily circuit switched networks. In a circuit switched network, each call establishes a dedicated point-to-point connection through the network which, for instance, allows people at both ends of a telephone call to speak and listen at the same time.
A circuit remains open for the entire duration of a call even if no one is speaking, which means that a significant amount of circuit's bandwidth, or capacity to carry information, is wasted on silence, or meaningless data. In order to utilize the capacity more efficiently, circuit switched telecommunication networks have made use of time division multiplexed (TDM) circuits to interconnect network switches. In TDM, analog signals are digitally coded and multiplexed in time over circuits at a constant bit rate.
The wide spread use of computers in the last decades has led to the development of additional types of networks. These networks have been configured for the purpose of data communications and are primarily packet-switched networks. In a packet-switched network, a call may consist of a stream of data sent from one computer to another. The stream of data is divided up into packets before it enters the network. At the destination, the stream of data is re-assembled from the packets. Thus packet-switched networks typically do not allocate fixed resources to transmitters, but rather route packets of data on a best efforts basis using destination address information contained in packet headers, and network switches and routers.
A packet-switched call therefore does not require a dedicated connection through the network. Instead, packets from many different calls can share the same bandwidth. That is, packets from one call can be inserted into spaces between packets from other calls. For these reasons, packet-switched networks efficiently utilize much more network bandwidth than circuit-switched networks, making packet-switched networks particularly suited to handle large volumes of data.
Packet-switched networks are becoming more popular amongst network operators as they often provide better performance, and are more cost effective to install and maintain than equivalent circuit-switched networks. Moreover, for the above-mentioned reasons of performance and cost, many operators and leased line providers who provide bandwidth to service providers are moving towards replacing TDM sources with packet networks. In many cases, switch-to-switch communications will be provided entirely over packet networks. However, it is likely that for many years to come, some operators will continue to rely upon TDM circuits to provide all or at least a part of the networks. This will bring about a constant need for interworking methods and systems between packet networks and TDM systems.
Packet-switched networks, however, normally do not work well for time critical transmissions such as voice. For instance, in packet-switched networks, packets may experience delay variations while traveling through the network. As a result, packets are rarely received at a constant bit rate. In data communications, delay variations between packets usually do not matter. A computer can just wait for a complete set of packets to arrive before processing the data. For time critical transmissions, however, delay variations can have a significant impact on the quality of the call. In such case, circuit-switched networks like TDM are generally better suited for constant bit rate, time critical transmissions such as voice communication.
In general, TDM links are synchronized circuits with a constant (transmission) bit rate governed by a service clock operating at some pre-defined frequencies. In contrast, in a packet network there is no direct link between the frequency at which packets are sent from an ingress port of the network and the frequency at which they arrive at an egress port of the network. In order to provide a TDM circuit emulation, there must be provided at the ports of the packet network an interworking between the TDM links and the packet network in such a way that the TDM link at the ingress side is synchronized with the TDM link at the egress side. That is to say, that the TDM service frequency at the customer premises on the ingress side must be exactly reproduced at the egress side of the packet network. The consequence of any long-term mismatch in these frequencies will be that the queue, for example, in a buffer memory etc., at the egress of the packet network will either fill-up or empty, depending upon whether the regenerated clock is slower or faster than the original clock, causing loss of data and degradation of the service. Also, unless the phase of the original clock is tracked by that of the regenerated clock, a lag in frequency tracking will result in small but nonetheless undesirable changes to the operating level of the queue at the egress.